Intermediate layer lithography

ABSTRACT

An isotropic or partially isotropic etch shrinks lithographically patterned photoresist (211, 212) to yield reduced linewidth patterned photoresist (213, 214) with a buried antireflective coating also acting as an etchstop or a sacrificial layer. The reduced linewidth pattern (213, 214) provide an etch mask for subsequent anisotropic etching of underlying material such as polysilicon (206) or metal or insulator or ferroelectric.

CROSS-REFERENCE TO RELATED APPLICATIONS

The following coassigned and cofiled U.S. patent applications discloserelated subject matter: application Ser. Nos. 08/680,340, pending filedJul. 12, 1996 and 08/678,847, pending filed Jul. 12, 1996.

BACKGROUND OF THE INVENTION

The invention relates to electronic semiconductor devices, and, moreparticularly, to fabrication methods for such devices.

Semiconductor integrated circuits with high device density requireminimum size structures such as short gates for field effect transistors(FETs), small area emitters for bipolar transistors, and narrowinterconnection lines between devices. The formation of such polysiliconor metal structures typically involves definition of the locations ofsuch structures in a layer of photoresist on a layer of polysilicon ormetal by exposure of the photoresist with light passing through areticle containing the desired structure pattern. After exposure anddevelopment of the photoresist, the underlying layer of polysilicon ormetal is anisotropically etched using the patterned photoresist as theetch mask. Thus the minimum polysilicon or metal linewidth equals theminimum linewidth that can be developed in the photoresist. Currentoptical steppers expose the photoresist using light of wavelength 365 nm(called I-line after the corresponding emission line in a high-pressuremercury arc lamp used to generate the light), and pattern linewidths inphotoresist of less than about 0.30 μm with a standard deviation of lessthan about 0.01 μm cannot be satisfactorily generated with I-linelithography.

FIGS. 1a-c illustrate a known method to create sublithographicpolysilicon gate structures and includes minimal geometry patterningphotoresist on a polysilicon layer (FIG. 1a), isotropically etching thephotoresist to reduce linewidth (FIG. 1b), and anisotropically etchingthe polysilicon with the reduced linewidth photoresist as etch mask(FIG. 1c). This approach has problems including contamination of thepolysilicon.

The use of a photoresist mask for anisotropic etching polysilicon gatescan leave residual ridges of hardened photoresist on the edges of thepolysilicon gates after the etch. Plasma etch species harden thephotoresist sidewalls during the polysilicon etch, and the subsequentoxygen plasma photoresist strip may not fully remove the ridges; FIG.1d. Also, separate wet etches to strip the ridges may be used but lackrobustness with respect to modifications. Any ridge residue willcarbonize during later heat treatments and impede formation of titaniumdisilicide (TiSi₂) in a self-aligned gate siliication process. Thussimple and complete removal of photoresist residue is a problem.

SUMMARY OF THE INVENTION

The present invention provides sublithographic patterns by use of anintermediate layer between photoresist and material to be etchedtogether with lateral etching of either a lithographically definedphotoresist pattern or the intermediate layer to shrink the linewidth.The intermediate layer acts as (1) a antireflective layer forphotoresist exposure, (2) as an etchstop or as a sacrificial layer forthe subsequent lateral etch, and/or (3) a liftoff layer for removal ofhardened photoresist residue.

Advantages of the invention include a simple method for sublithographicpatterns and robust photoresist removal.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings are heuristic for clarity.

FIGS. 1a-d show a known sublithographic pattern method.

FIGS. 2a-h illustrate first preferred embodiment method of photoresistpatterning in cross sectional elevation and plan views.

FIG. 3 is a cross sectional elevation view after an anisotropic etchusing the first preferred embodiment.

FIGS. 4a-d illustrate a second preferred embodiment method ofphotoresist patterning.

FIG. 5 suggests etch selectivity.

FIG. 6 illustrates a polysilicon etch.

FIGS. 7a-b show in a cross sectional elevation views a third preferredembodiment method.

FIGS. 8a-e are cross sections of a fourth preferred embodiment method.

FIG. 9a-d illustrate in cross sectional elevation views a fifthpreferred embodiment method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Overview

The preferred embodiment methods of sublithographic pattern creationinsert an intermediate layer between photoresist and the material to bepatterned and use the following steps: first expose and develop apattern in the photoresist with a minimal linewidth and then laterally(e.g., isotropically) remove photoresist or intermediate layer or bothto uniformly shrink the intermediate layer to a subminimal linewidthwhich then provides the etch mask for the material to be patterned. Theintermediate layer may provide (1) an antireflection function duringphotoresist exposure, (2) an etchstop or a sacrificial layer to protectunderlying material layer during the subsequent lateral removal, and/or(3) an etch residue liftoff layer after the material has been patterned.

The sublithographic patterning and residue liftoff can be created overmaterials such as polysilicon, metal, insulator, ferroelectric, and soforth. The sublithographic pattern may define minimal sized items suchas gate length and interconnection linewidth for integrated circuits.

First preferred embodiment

FIGS. 2a-h illustrate the first preferred embodiment photoresistpatterning method as could be used to form a mask for gate levelpolysilicon etching. In particular, begin with monocrystalline siliconsubstrate 202 having (100) orientation and typically with both p and ntype doped well regions for fabrication of devices plus also isolationoxides 203, gate oxide 204 with thickness typically 6-10 nm plus gatelevel polysilicon layer 206 with thickness typically 300-500 nm andeither doped or undoped or doped only in certain portions. Then proceedwith the following steps:

(1) Sputter deposit a 55 nm thick layer 208 of titanium nitride (TiN)onto polysilicon 206. TiN layer 208 acts as a buried antireflectivecoating ("BARC") for I-line lithography; that is, TiN strongly absorbs365 nm wavelength light. Without TiN or some other BARC, the underlyingpolysilicon 206 would reflect exposure light penetrating overlyingphotoresist and cause interference which makes the photoresist's degreeof exposure depend upon location because the photoresist thicknessvaries over protuberances such as isolation oxide 203.

(2) Spin on roughly 1 μm thick I-line photoresist layer 210 onto TiNBARC 208; the thickness of layer 210 depends on the underlyingtopography. I-line photoresist may be made of cyclized polyisoprenepolymers with azide sensitizers. Softbake photoresist 210 if desired.See FIGS. 2a-b for cross sectional elevation and plan views.

(3) Expose photoresist 210 with an I-line lithography system to define apattern with minimum linewidth of 0.33 μm. Then develop exposedphotoresist 210 and bake to yield patterned photoresist portions 211 and212 as illustrated in FIGS. 2c-d in cross sectional elevation and planviews. The width denoted "W" may be a minimum line width such as 0.33μm. See FIGS. 2c-d with FIG. 2c being the section along line C--C inplan view FIG. 2d.

(4) Apply an isotropic etch to remove ΔW of photoresist patterns 211-212to yield photoresist patterns 213-214, but the etch only removes anegligible amount of TiN 208. This isotropic etch may be a plasma etchwith 80% helium and 20% oxygen at a pressure of 1.5 mTorr which removesphotoresist at a rate of 160 nm/min. Thus a 15 second etch would remove0.04 μm of photoresist and reduce a 0.33 μm linewidth down to a 0.25 μmlinewidth. See FIGS. 2e-f which show the etched photoresist patterns213-214 defining a linewidth of W-2ΔW with solid lines together with theoriginal photoresist patterns 211-212 defining a linewidth of W withbroken lines.

(5) Apply an anisotropic etch to remove the exposed portions of TiNlayer 208 and complete the etch mask for etching polysilicon 206. Ahelicon plasma etcher with chlorine at a pressure of 6 mTorr will etchTiN at about 200 nm/min, so an etch of roughly 15 seconds will removeexposed TiN to leave TiN portions 217-218. This etch will also etchpolysilicon at roughly the same rate, but stopping in polysilicon 206 isnot critical because polysilicon 206 will be anisotropically etched nextanyway. FIGS. 2g-h illustrate the final photoresist patterns 213-214 onunderlying TiN portions 217-218 which form the mask with W-2ΔW minimumlinewidth to be used for anisotropic etching of polysilicon 206.

The anisotropic etching of polysilicon 206 then proceeds with a heliconexcited plasma from a gas mixture of Cl₂, HBr, and He/O₂ (80%/20%) at apressure of about 6 mTorr and using the photoresist patterns 213-214 asetch mask. The Br provides sidewall passivation to insure anisotropy.Cl₂ /HBr/He--O₂ plasma may etch polysilicon about 300 times faster thanoxide, and an overetch on oxide 204 will only remove a minimal amount ofoxide; see FIG. 3. A final oxygen plasma strips the photoresist plus achlorine plasma or an SC1 rinse (NH₄ OH+H₂ O₂ +H₂ O solution) strips theTiN from the etched polysilicon without affecting either the polysiliconor the exposed gate oxide.

Second preferred embodiment

FIGS. 4a-d illustrate the second preferred embodiment photoresistpatterning method as could also be used to form a mask for gate levelpolysilicon etching. In particular, again begin with monocrystallinesilicon substrate 402 having (100) orientation with isolation oxides403, gate oxide 404 with thickness 6 nm plus gate level polysiliconlayer 406 with thickness 400 nm. Then proceed with the following steps:

(1) Spin on a 200 nm thick layer 408 of organic BARC onto polysilicon406. That is, organic BARC layer 408 strongly absorbs 365 nm wavelengthlight. Organic BARC 408 may be a polymer with attached dye groups whichprovide the absorption but without change in polymer bonds; for example,polyamic acid polymers and copolymers. As previously noted, without somesort of BARC, the underlying polysilicon 406 would reflect exposurelight penetrating overlying photoresist 410 and cause interference whichwould make the photoresist's degree of exposure depend upon locationbecause the photoresist thickness varies.

(2) Spin roughly 1 μm thick photoresist layer 410 onto BARC layer 408;the thickness of layer 410 depends on the underlying topography. SeeFIG. 4a for a cross sectional elevation view.

(3) Expose photoresist 410 with an I-line lithography system to define apattern with minimum linewidth of 0.30 μm. Then develop the photoresistand bake to yield patterned photoresist portions 411 and 412 asillustrated in FIG. 4b. The width denoted "W" may be a minimum linewidthsuch as 0.30 μm.

(4) Etch using a mixture of CHF₃ /CF₄ /O₂ or CHF₃ /O₂ at a pressure of25-75 mTorr in a parallel plate plasma etcher to anisotropically removethe exposed portion of BARC layer 408. This etch also removesphotoresist isotropically with a rate dependent upon the CHF₃ to CF₄ratio: a mixture of CHF₃ and O₂ removes photoresist (which may be basedon a polymer of isoprene) at roughly the same rate as it removes BARC,whereas CF₄ and O₂ does not rapidly remove photoresist. FIG. 5 suggeststhe photoresist-to-BARC etch ratio as a function of gas mixture. Thus byselecting the gas mixture, ΔW of photoresist patterns 411-412 can beremoved during the BARC etch for any desired ΔW from 0 up to 200 nm toyield photoresist patterns 413-414 with linewidth W-2ΔW. For example,the lithographically defined linewidth of 0.30 μm can be reduced down toa 0.25 μm linewidth during the BARC removal with overetch if the lateraletch rate of photoresist is about 1/10 the vertical etch rate of BARC.See FIGS. 4c-d which show the lateral and vertical etches and the etchedphotoresist patterns 413-414 defining a mask with linewidth of W-2ΔW foretching polysilicon 406.

The etching of polysilicon 406 then proceeds with a plasma from a gasmixture of SF₆ plus HBr using the photoresist patterns 413-414 as etchmask. The Br provides sidewall passivation for anisotropy. Further, theBARC etch of step (4) deposits material 450 on the BARC sidewall asillustrated in FIG. 4d; and during the polysilicon etch this sidewallmaterial migrates down the forming polysilicon sidewall as illustratedin FIG. 6 and limits microtrenching at the sidewall base. Use a Cl₂/HBr/He--O₂ plasma etch to finish and overetch because this mixtureetches polysilicon about 300 times faster than oxide, and an overetch onoxide 404 will only remove a minimal amount of oxide. A final oxygenplasma strips the patterned photoresist plus BARC.

Various anisotropic polysilicon etches have differing amounts ofintrinsic linewidth reduction. Consequently, use of the second preferredembodiment allows compensation for the polysilicon etch by adjusting theBARC etch gas mixture so that the total linewidth reduction (photoresistlinewidth reduction by BARC etch plus linewidth reduction by polysiliconetch) remains constant.

Third preferred embodiment

The third preferred embodiment again uses an isotropic etch to reducethe minimum linewidth of a photoresist mask with TiN antireflectivecoating in the case of a metal etch. In particular, aluminuminterconnections often have TiN cladding to act as diffusion barriersand electromigration suppressors. Thus FIG. 7a shows silicon substrate702 with insulated gate 704 and planarized oxide insulation 706 having atungsten filled via 708 connecting down to the source/drain of the FETwith gate 704 plus layer 712 of aluminum clad by layers 710 and 714 ofTiN.

Next, spin on photoresist 720 and expose with masked I-line light for apattern with linewidth W; top TiN cladding 714 acts as theantireflective coating. Develop photoresist 720 and then apply an oxygenplasma etch to shrink patterned photoresist 720 linewidth to W-2ΔW usingthe top TiN cladding 714 as an etchstop; see FIG. 7b showing thepatterned photoresist shrinkage.

Then apply a chlorine based anisotropic etch to remove TiN 714, Al 712,and TiN 710 not masked by patterned photoresist 720. Strip the patternedphotoresist 720 with an oxygen plasma. In this case the structural layerof TiN 714 also acted as the buried antireflective coating and thephotoresist linewidth shrink etchstop.

Fourth preferred embodiment

FIGS. 8a-d illustrate the fourth preferred embodiment method as couldalso be used to form a mask for gate level polysilicon etching. Inparticular, again begin with monocrystalline silicon substrate 802having (100) orientation with isolation oxides 803, gate oxide 804 withthickness 6 nm plus gate level polysilicon layer 806 with thickness 400nm. Then proceed with the following steps:

(1) Deposit a 200 nm thick TiN layer 808, which acts as I-line BARC,onto polysilicon 806. TiN deposition may be by sputtering Ti in a N₂plasma or sputtering TiN. As previously described, BARC 808 limitsreflective interference in an overlying photoresist layer which wouldotherwise make the photoresist's degree of exposure depend upon locationbecause the photoresist thickness varies.

(2) Spin roughly 1 μm thick photoresist layer 810 onto BARC layer 808;the thickness of layer 810 depends on the underlying topography. SeeFIG. 8a for a cross sectional elevation view.

(3) Expose photoresist 810 with an I-line lithography system to define apattern with minimum linewidth of 0.30 μm. Then develop the photoresistand bake to yield patterned photoresist portions 811 and 812 asillustrated in FIG. 8b. The width denoted "W" may be a minimum linewidthsuch as 0.30 μm.

(4) Apply an anisotropic etch to remove the exposed portions of BARClayer 808; see FIG. 8c. For TiN BARC a helicon plasma etcher withchlorine at a pressure of 6 mTorr will etch TiN at about 200 nm/min, soan etch of roughly 60 seconds will remove exposed TiN to leave BARCportions 821-822. This etch will also etch polysilicon at roughly thesame rate, but stopping at polysilicon 806 is not critical becausepolysilicon 806 will be anisotropically etched in step (6).

(5) Apply a timed isotropic etch to laterally remove about 0.025 μm ofBARC 821-822 to form narrowed BARC portions 823-824 of minimal width0.25 μm; see FIG. 8d showing the minimal linewidth of W-2ΔW. Theisotropic etch for TiN BARC may be a wet etch of dilute H₂ O₂ whichetches TiN at about 5 nm/min, so this would be a 5 minute etch. Notethat overlying photoresist 811-812 limits the amount of BARC exposed toany etchant and thereby greatly diminishes proximity effects to insure auniform removal of the lateral 0.025 μm of BARC over an entire wafer.Similarly, an isotropic plasma etch could be used. The narrowed BARC823-824 forms the final etch mask with W-2ΔW minimum linewidth to beused for anisotropic etching of polysilicon 806.

(6) First strip overlying photoresist 811-812 with an oxygen plasma, andthen anisotropically etch polysilicon 806 with the BARC 823-824 as etchmask. Note that the thickness of BARC 823-824 permits use of a somewhatnonselective anisotropic polysilicon etch; that is, a plasma etch mayalso remove the BARC provided that it removes the polysilicon at leasttwice as rapidly. See FIG. 8e. Lastly, strip the BARC to leave thesublithographically patterned polysilicon.

Note that use of an organic BARC would also be possible provided thatthe photoresist could be removed without also removing the BARC.

Fifth preferred embodiment

FIGS. 9a-d illustrate the fifth preferred embodiment method as couldalso be used with any of the foregoing preferred embodiment linewidthreduction methods or could be used without them. The fifth preferredembodiment uses the intermediate layer (possibly BARC) as a liftoff toremove the overlying photoresist or photoresist residue according to thefollowing steps.

(1) Begin with patterned photoresist 911-912 on 50 nm thick TiNintermediate layer portions 917-918 which, in turn, lie on 300 nm thickpolysilicon 906. The TiN acted as the BARC for the patterning of thephotoresist and, optionally, photoresist 911-912 may have beenisotropically etched to shrink linewidth analogous to the structure ofFIG. 2g. See FIG. 9a.

(2) Anisotropically etch polysilicon 906 with a Cl and Br based plasmawith photoresist 911-912 plus TiN 917-918 as the etch mask. The etchplasma also forms hardened photoresist portions 913-914 from thesidewalls of photoresist 911-912. See FIG. 9b.

(3) Dissolve TiN 917-918 in a solution such as SC1 (1 part 29% NH₄ OH, 1part 30% H₂ O₂, and 6 parts H₂ O); this also lifts off photoresist911-912 along with hardened sidewall portions 913-914. See FIG. 9c.Optionally, prior to dissolution of TiN 917-918, ash photoresist 911-912with an oxygen plasma and then dissolve TiN 917-918. This prior ashingexposes another surface of TiN 917-918 for quicker dissolution and stillpermits liftoff of the hardened sidewall portions 913-914 which theoxygen plasma fails to remove. See FIG. 9d which shows the structureafter photoresist ashing but prior to the TiN dissolution.

Intermediate layer 917-918 may have been an organic BARC layer, and themethod would follow the same steps with BARC dissolution by organicsolvent providing the liftoff of the hardened sidewall portions 913-914.However, organic BARC sidewalls would likely also become hardened, sodissolution may require a particular solvent adapted to the type of BARCused. And using an intermediate layer under photoresist for liftoff ofhardened sidewalls also applies to etching metal levels, analogous tothe third preferred embodiment, and to etching vias through insulators.

Modifications and variations

The preferred embodiments may be varied in many ways while retaining oneor more of the features of use of an intermediate layer which may act asa buried antireflective coating, as an etchstop or sacrificial layer forlinewidth reduction, and as a liftoff for overlying photoresist orresidue or other material.

For example, the photoresist of the first, second, and third preferredembodiments could be stripped and the patterned BARC alone used as theetch msk provided the etch is selective enough with respect to the BARC.The isotropic etch to reduce photoresist linewidth could be somewhatanisotropic provided sufficient photoresist remains. The layerthicknesses and linewidths and etch chemistries and conditions could allbe varied. Further, the preferred embodiment descriptions all usedI-line lithography, whereas with other exposure wavelengths using thesame or different photoresist and antireflective coatings the sameapproach works. Further, variations could use a single wafer heliconplasma etcher or other types of plasma etchers including batch RIE, ECRRIE, and inductively coupled plasmas.

What is claimed is:
 1. A method of lithography, comprising the stepsof:(a) providing a bottom layer to be patterned; (b) forming anintermediate layer over said bottom layer; (c) forming a radiationsensitive top layer over said intermediate layer; (d) patterning saidtop layer with radiation to form a patterned top layer; (e)simultaneously laterally removing portions of said patterned top layerand vertically removing exposed portions of said intermediate layer toform a reduced patterned top layer on a patterned intermediate layer;and (f) removing portions of said bottom layer using said patternedintermediate layer as at least a portion of a mask.
 2. The method ofclaim 1, wherein:(a) said intermediate is made of organic polymer; and(b) said top layer is made of photoresist.
 3. The method of claim 2,wherein:(a) said simultaneously removing is by plasma etching.
 4. Themethod of claim 3, wherein:(a) said bottom layer is polysilicon; and (b)said removing portions of said bottom layer is by anisotropic plasmaetching.
 5. A method of sublithographic patterning, comprising the stepsof:(a) providing a bottom layer to be patterned; (b) forming a buriedantireflective coating (BARC) layer on said bottom layer, said BARClayer absorptive of radiation of a first wavelength; (c) forming aphotoresist layer on said BARC layer, said photoresist layer exposibleby radiation with said first wavelength; (d) patterning said photoresistlayer with radiation including said first wavelength to form a firstpatterned layer of photoresist with minimum linewidth of W; (e)simultaneously etching said first patterned layer to remove an amount ΔWfrom all surfaces of said first patterned layer in a direction along asurface of said BARC layer and etching said BARC layer to remove exposedportions to form a second patterned layer of photoresist with minimumlinewidth of W-2ΔW on a patterned BARC layer; and (g) anisotropicallyetching said bottom layer using said second patterned layer andpatterned BARC layer as etch mask.